Images older than all-in-one-sandbox 1.9.x have no /v1/bash/* routes, so
every env-bearing command (skills declaring required-secrets) surfaced a
raw nginx 404 that the model kept retrying. Detect the 404, remember the
capability gap per sandbox instance, and return an actionable error that
names the minimum image version and the sandbox.image remediation.
No fallback through the legacy shell path on purpose: /v1/shell/exec has
no env parameter, and every workaround puts the secret values back into
the command string or on disk — the exact leak surfaces the
request-scoped secrets design closed.
Closes#3921
48477d868b